Petroleum geology of the basal Niobrara sandstone West Puerto Chiquito field Rio Arriba County New Mexico USA

Abstract

The basal Niobrara sandstone in the eastern San Juan Basin is an important oil producing reservoir. The West Puerto Chiquito (WPC) Field has produced approximately 18 million barrels of oil from Fort Hays age equivalent sandstones. Most of the Late Cretaceous oil producing stratigraphic traps that have been identified in the Rocky Mountain region are associated with unconformities. The boundary between the Carlile Shale and Niobrara is a well-known regional unconformity that has been extensively studied on the west side of the San Juan Basin in association with large oil fields such as Bisti and Horseshoe. A study of well log correlation, outcrop, core and source rock analysis was conducted in order to identify the resource potential of the Niobrara sandstone in the eastern part of the San Juan Basin. The Niobrara Sandstone is encased within 2000 feet of organic-rich, thermally mature Mancos source rock. Core analyses, including petrographic thin sections help to characterize the porosity and permeability of this tight reservoir. Surface lineaments were studied in order to identify reservoir connectivity. Structural mapping indicates that relative structurally high folds along the steeply dipping eastern San Juan Basin margin are associated with higher volume oil production. Log analysis also indicates that resistivity over 62 Ohm-m in the Niobrara C sandstone interval are the most productive. Geochemical analysis shows that total organic content (TOC) ranges from 1-4 wt. % and the highest TOCs are located at the top of the Mancos Shale and also within 200 feet above the Niobrara sandstones. Source rocks are mature with Tmax values measured from 435°C to 450°C and calculated vitrinite reflectance ranges from 0.6 to 1.0. Measured S2 fractions were low due to the high level of maturity. Type II and III are the primary kerogen types with some type IV which is overrepresented by high levels of kerogen maturation. Rock work comparison of outcrop to the subsurface indicated that the subsurface cores are comprised of finely laminated sandstone and mudstone interbeds that are conformable. Outcrop sandstones are massive continuous beds with sharp mudstone contacts above and below the Niobrara A, B and C sandstone intervals. The sandstones are interpreted to be offshore sand bars within an isolated embayment. Petrographic thin sections indicate that the cored Niobrara sandstone intervals are very fine-grained calcareous sandstones with fine lamination of organic material and marine fossil fragments. There are no apparent unconformities observed in cores or in outcrop. Basin modeling indicates that peak oil generation occurred at 59 Ma and primary gas generation began at 50 Ma. Production observations reveal that the highest production coincides with surface lineaments that indicate regional subsurface connectivity. WPC oil is produced through solution gas drive and the updip trap and seal are likely related to a maturity boundary along with an updip fault-seal. The Niobrara sandstones at West Puerto Chiquito are an unconventional resource that represent an opportunity to horizontally drill and downspace development based on the observed reservoir characteristics.